The present invention relates broadly to a power supply control apparatus, and in particular to a power supply sequency apparatus. It is quite common to simultaneously power electronic devices with positive and negative power supply sources. However, where one of the power supply sources is shut down either intentionally or due to some overload or safety condition, the other power supply source is not necessarily shut down. Therefore, an electronic device which is powered by these supplies in the situation, is subjected to an unusual condition. The shutting down of the one of the voltages due to an overload or whatever thereof without shutting down the other voltages is undesirable since a portion of the electronic circuitry will continue to be energized when a portion will be shut down. The partial energization of the common circuitry may lead to destruction of part of the circuitry and is therefore quite undesirable.
In the prior art, the approach to power supply sequencing problems has seen the use of double pole relays assuming in the situation with a two supply device, to apply the power simultaneously to the critical component. Such an arrangement, however, requires some type of sense circuitry to detect the failure at one of the supplies and deenergize or prevent the energization of the relay in case of a failure or other absence of either supply. It is clear that obviously the additional sense circuitry complicates the overall system, is expensive, power consuming, lowers the overall reliabiity and in general the relay approach is not compatible with modern integrated circuitry. Bipolar transistor switches have also been used in place of relay contacts but their application suffers from most of the same complications mentioned above.